We finally know what caused the refinery blast that rocked Philadelphia

The June 21 fire at the Philadelphia Energy Solutions refining complex was likely caused by a faulty pipe, a preliminary report from the U.S. Chemical Safety and Hazard Investigation Board has found.

The fire, which raged for more than 24 hours, released 5,239 pounds of deadly chemicals into the air.

The blaze broke out early in the morning on June 21st when an elbow pipe most likely failed, allowing flammable fluid containing propane and other chemicals to escape. The leaking fluid quickly turned into a vapor cloud, which ignited shortly thereafter. As the fire raged, at 4:15 a.m. the first of three explosions occurred. The second was four minutes later, and the third, which was by far the most powerful, occurred at 4:22 a.m.

During the last and greatest explosion, a vessel within a unit containing highly flammable hydrocarbons ruptured, hurling fragments into the air. The blast was so powerful that a 38,000 pound barrel was launched 2,100 feet across the Schuylkill river, where it landed on the opposite bank.

The fire illuminated the sky and sent shock waves for miles around the complex. Homes in South Philadelphia shook as debris rained down.

Based on stamp marks found on the faulty piping, investigators estimate that it was installed around 1973. The elbow pipe that likely started the fire was so worn down that it was just 0.012 inches thick. As the report noted, this is about half the thickness of a credit card.

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According to estimates from Philadelphia Energy Solutions, 5,239 pounds of deadly hydrofluoric acid was released during the fire and subsequent explosion. The company estimated that it contained about 1,968 pounds within the refining unit by using water spray, which means that the larger share, or roughly 3,271 pounds, escaped into the atmosphere.

Chemical Safety Board interim executive director Kristen Kulinowski said it was lucky that there were “no serious injuries or fatalities.”

Five workers reported minor injuries in connection with the blast, and the investigation said it was unaware of any health impacts from the hydrofluoric acid release.

Five days after the incident Philadelphia Energy Solutions announced that it would be shutting down the plant, which had been the largest refinery on the East coast. The following month, on July 22, the company filed for bankruptcy.

The federal investigation is ongoing.

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